Semiconductor photodetectors play an important role in the development of optical communication. They function as converters, switching light signals into electrical signals, and allowing data to be transmitted across the optical platform to the electrical platform. Semiconductor photodetectors can also be used as monitoring tools, enabling one to observe the power level of a particular signal, providing feedback to the system for power stabilization and control.
There are many different forms of photodetectors. PIN photodetectors are one of the most commonly used types of photodetectors. This kind of photodetector has an intrinsic region sandwiched between a p+ region and a n+ region, in which its high resistivity causes any applied bias to drop almost entirely across the layer. Thus, all photogenerated carriers (electron-hole pairs) will be accelerated by the bias to the two opposite ends. The detection scheme could either be surface incident or waveguide incident. In either case, the detection response time is limited by the applied bias, the depletion layer width, and the fact that the electrons and holes need to travel a long distance to reach both ends of the contact.
Metal-semiconductor-metal (MSM) detectors are another form of photodetector. They are made by forming two Schottky contacts on an undoped semiconductor layer e.g. InGaAs. In operation, the incident light is absorbed by the active layer and generates electron-hole pairs; this would increase the current in the external circuit, which in turn would be detected by the contact pads. Due to the different rates of the electrons and holes reaching the contacts, this kind of detector produces gain, and due to the short distance from the electrons/holes to the contacts, the speed of the detector is normally faster as compared to PIN detectors. MSM detectors are generally used with a surface incident scheme with the metal contacts directly located above the active region.
In this disclosure, we propose a new high-speed waveguide photodetector scheme with an MSM configuration.
According to the present invention, a waveguide photodetector comprises:
a waveguide having a III-V ridge structure including an active layer;
a semiconductor layer deposited on top of the ridge, the semiconductor layer having a higher refractive index than the waveguide structure; and,
metal detector electrodes on the surface of the higher refractive index semiconductor layer.
wherein the ridge increases in width along at least a portion of the length of the waveguide such that light passing through the active layer of the waveguide couples up into the higher refractive index semiconductor layer.
The present inventors have found that, by placing a higher refractive index semiconductor layer on top of the ridge of a ridge structure waveguide, as the width of the ridge is increased, light preferentially couples up into the higher refractive index layer from the active layer of the waveguide structure. Therefore, by placing electrodes complying with the MSM detection scheme on top of the higher refractive index layer, light from the waveguide can be detected.
Preferably, the waveguide is an InP/lnGaAsP waveguide. Preferably, the higher refractive index layer is undoped or lightly doped InGaAs.
In a preferred embodiment, the ridge increases in width continuously at the portion where the width increases.
The continuous width increase is an increase in width which is smooth (i.e. has no discontinuity). The increased width region has an enhanced optical coupling efficiency.
The detector electrodes may include a plurality of interdigitated metal fingers.
Advantageously, the waveguide photodetector of the present invention can be disposed within a photodetector branch of an optical tap.
According to a further aspect of the present invention, there is provided a waveguide photodetector comprising: an elongated waveguide having a III-V ridge structure including an active layer; a semiconductor layer deposited on top of the ridge, the semiconductor layer having a higher refractive index than the waveguide structure; and, metal detector electrodes on the surface of the higher refractive index semiconductor layer, wherein the ridge has a first width in a first region of the waveguide and the ridge has a second width in a second region of the waveguide, the second width being greater than the first width, and wherein the detector electrodes are provided adjacent to the second region of the waveguide such that light passing through the active layer of the waveguide couples up into the higher refractive index semiconductor layer.
The second width is preferably at least 10 xcexcm.
In addition to a first and a second region, the ridge may further comprise an intermediate region between the first region and the second region in which the width of the ridge increases continuously along the length of the intermediate region.